It is known to separate oxygen from air using an adsorbent which has the ability to effect a separation as between the two major components of air by virtue of its greater adsorption of nitrogen than of oxygen. Naturally occurring zeolites will selectively adsorb nitrogen and other constituents of air in preference to oxygen. Synthesis of zeolites has greatly enhanced this selectivity enabling oxygen of purities from 93% or better to be produced by PSA techniques.
In operation, a bed of the adsorbent is put through a cycle which includes an adsorption step during which time air is pumped through the bed under pressure and most of the nitrogen and a proportion of the oxygen and substantially all of the carbon dioxide and water vapor in the feed air are adsorbed and an oxygen rich product gas is supplied from the outlet of the bed; and a desorption step during which time the outlet of the bed is closed, the bed is vented to atmospheric pressure usually through its inlet and/or evacuated through its inlet so that the adsorbed gases are substantially removed from the bed thereby preparing it for the next adsorption step.
In practice, it is usual for two adsorbent beds to be employed and operated on similar cycles which are sequenced to be out-of-phase with one another by 180.degree. so that while one bed is on its adsorption step, the other bed is on its desorption step and vice versa. In a simple process having only adsorption and desorption steps the efficiency of the oxygen production is low. Additional steps are usually incorporated to achieve better purities and a more efficient process and one of these additional steps can be a purge step in which product quality gas from the bed undergoing adsorption is diverted to the top of the bed undergoing desorption said oxygen rich gas purging or displacing nitrogen in the desorbing bed.
However, any PSA process that uses a purge step to lower the partial pressure within a bed during regeneration suffers an imbalance from many possible causes such as different quantities or qualities of adsorbents in each bed, different pressures or differing restrictions to each bed. The degree of difficulty in trying to balance the beds is time consuming and will only be correct at the time of adjustment. Variations in ambient atmospheric conditions can deviate the plant operation from its optimum set parameters.
It is an aim of the present invention to so modify a known PSA plant for the production of oxygen that an optimum purge can be used at different production rates regardless of ambient or process conditions such that regeneration can be obtained from each bed at its particular regeneration requirement.